Remove redundant initializations

There are certain parts of code that laboriously initialize every field 
of a struct to (some spelling of) zero, even though the whole struct was 
just zeroed (by makeNode() or memset()) a few lines earlier.  Besides 
being redundant, I find this hard to read in some situations because 
it's then very hard to tell what is different between different cases or 
branches.  The attached patch cleans up most of that.  I left alone 
instances where there are (nontrivial) comments attached to the 
initializations or where there appeared to be some value in maintaining 
symmetry.  But a lot of it was just plain useless code, some clearly 
copy-and-pasted repeatedly.

Note 
<https://www.postgresql.org/message-id/flat/4c9f01be-9245-2148-b569-61a8562ef190@2ndquadrant.com> 
where we had a previous discussion about trimming down useless 
initializations to zero.